Climate Science Glossary

Term Lookup

Settings

Use the controls in the far right panel to increase or decrease the number of terms automatically displayed (or to completely turn that feature off).

Term Lookup

Term:

Settings

Beginner Intermediate Advanced No DefinitionsDefinition Life:

All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Scientists think the risks of climate change increase as temperatures rise. Two degrees isn't a 'safe' level of climate change, but nor is it a red line with only chaos beyond, as we'll see in part three.

It is a readily understood and useful marker of how we're doing at limiting dangerous climate change that has helped focus minds on the scale of the challenge. It's also what the world's governments have committed to achieving.

So will we manage to limit warming to two degrees above pre-industrial? We take a look at what it will take to stay below two degrees, how things are going so far, how experts say we should proceed and what we'd need to do if we wanted to follow their advice.

1. What we would need to do to stay below two degrees

The world has already warmed by 0.85 degrees celsius above the pre-industrial average and if emissions stay high we're on course for more like three to five degrees by 2100,according tothe latest Intergovernmental Panel on Climate Change (IPCC) reports.

Broadly speaking, however, scientists say it is still theoretically possible to limit warming to two degrees as long as we stick within a fixedcarbon budget. This is the total amount we can emit from the beginning of the industrial revolution until the day we stop adding carbon to the atmosphere.

So how big is the budget? It is likely that we'll stay below two degrees as long as we emit no more than about 2,900 billion tonnes of carbon dioxide, the IPCCsays. 'Likely' here means a 66 per cent chance.

We've already emitted 1,900 billion tonnes, leaving a remaining budget of just 1,000 billion tonnes that we can emit between now and forever. At current rates we'lluse that quota upwithin 21 years.

If we're willing to accept a higher risk of breaching the two degree target then our budget would be a bit bigger. It might last 33 years at current emissions rates, instead of 21 years.

If the earth isless sensitive to emissionsthan we thought, that would increase the budget too: we could emit more carbon and still stay below two degrees. But at current rates we would burn through that extra allowance inabout a decade.

Whether climate sensitivity is lower than thought or not, we don't have many years left to significantly cut emissions.

2. What we are actually doing

Unfortunately global emissions arecurrently increasing, not decreasing, so we're using up that budget ever more quickly. Emissions from the burning of fossil fuels and production of cement reached arecord highof 36 billion tonnes of carbon dioxide equivalent in 2013, 2.3 per cent above the 2012 level.

Emissions are expected to grow by 2.5 per cent and set a new record of 37 billion tonnes in 2014, putting the world on course to breach the two degrees budget even sooner than the calculation above would suggest.

Historical emissions (the black dots and line on the chart below) are tracking above the top end of the range of IPCC projections, putting the world on a pathway which will lead to around five degrees of warming by 2100 (red line).

In order to get back on track, emissions need to peak and then fall by between 40 and 70 per cent by 2050, the IPCC says, with unabated fossil fuel burning almost entirely phased out by 2100.

The speed of emissions cuts required depends on when the peak occurs. The longer we wait to peak emissions, the faster they'llhave to fall afterwardsto stay within the budget.

Forget for a moment that emissions today are still increasing at more than 2 per cent a year and imagine emissions have already reached a peak today. Emissions would need to startfalling at 5.5 per cent per yearfrom tomorrow, in order to stay within a two degrees budget.

That would require a never-before seen global effort to be sustained for a generation.

By way of comparison,the US cut emissionsnearly 7 per cent during 2009 as a combined result of a shift from coal to gas-fired power and falling demand after the global credit crunch.

The fall of communist Russia and ensuing economic collapse saw average emissions reductions of 8 per cent a year sustained over the five years to 1994. And France managed 5 per cent per year for half a decade at the height of its 30 year shift to nuclear, during which it went from one per cent nuclear-powered to 80 per cent.

Meeting the two degrees goal means the equivalent of France's peacetime, non-recessionary five-year feat would need to be sustained globally over the space of three and a half decades.

3. The recipe for success

Such global rates of emissions reduction can seem quite distant from the realities of day to day life. Fortunately there are plenty of practical recipes available that set out exactly what we would need to do to stick within the two degrees budget.

The IEAsayssticking to two degrees is "technically feasible, but requires a fundamental transformation of the global energy system". First on the list is action that could be taken now at no net economic cost. These cost-neutral changes to the way energy is used would get us 80 per cent of the way towards a two degrees target, the IEA says.

First, the world must more than double its energy productivity so that each million dollars of wealth requires the equivalent of just 85 tonnes of oil, compared to 184 tonnes today. Greater energy efficiency and an end to fossil fuel subsidies would help limit increases in energy use even as the world's population reaches nine billion.

Second, the world must stop burning so much coal and oil, shifting instead to lower carbon alternatives including gas, nuclear and renewables as the chart below shows.

Third, energy emissions must reach net zero by 2070. Fourth, transport oil use must be progressively replaced by biofuels, gas and electric vehicles. Finally hundreds of coal-fired power stations must close while millions of solar panels and hundreds of thousands of wind turbines must be built.

The Global Commission on Climate and the Economy'sNew Climate Economy report, published in September, offers a similar vision for lower-carbon energy use. Crucially it too argues that most of the changes needed to limit warming can be made atno net economic cost.

Hundreds of similar exercises have attempted to connect the top-down requirements of atmospheric greenhouse gas concentrations to the bottom-up decisions that can be taken on the ground by politicians, businesses and ordinary citizens. But some two degrees scenarios have been criticisedfor failing to convincingly make that leap between theory and practice.

There is a difference between plotting out a path to a two degrees world at a global level and realistically explaining how to achieve the transition on the ground.

One influential approach to bridging this gap has been the 2004 idea of " stabilisation wedges". Each stabilisation wedge was supposed to represent a known method for cutting emissions that was ready to be implemented straight away, such as reducing deforestation or building renewables.

If scaled up over 50 years, each stabilisation wedge would eventually reduce carbon emissions by a gigatonne per year. At the time it was thought that implementing seven wedges would be enough to stabilise carbon emissions and solve the climate problem.

The concept became popular because it made climate change seem tractable. It was even turned into aclimate wedge gamethat could be played from classrooms to boardrooms. Players could discuss and then choose their preferred wedges, whether it was coal to gas conversion in the electricity sector or more efficient use of heat.

4. Following the recipe

Breaking the problem down into easier to conceptualise chunks has its attractions. Another way to do this is to look at individual technologies and ask what each might contribute to a two degrees transition.

Carbon Brief has covered a bunch of studies of this sort over the past year and each, on its own, represents a sizeable challenge.

For instance, investments inenergy efficiencyneed to triple from today's level to $900 billion in 2035, the IEA suggests, and non-economic barriers to the deployment of efficiency measures must beovercome.

Renewables output then needs to double again to 17,973 terawatt hours in 2040 to claim a greater than 50 per cent share of electricity generation. As part of that surge, solar power capacity needs toincreasefrom 139 gigawatts installed today to 5,656 gigawatts in 2050, enough to provide 27 per cent of power needs.

We used data from the IEA's recent World Energy Outlook to map the changes in electricity generation capacity that would be needed to limit warming to two degrees. The map shows how fast renewable capacity must increase and conversely how far coal capacity must fall in the world's major countries and regions.

The map clearly illustrates perhaps the most daunting challenge, also set by the New Climate Economy, to rapidly reduce emissions from coal-fired power. To get on a two degrees path carbon emissions from coal need to be cut by two-thirds of today's levels by 2040.

That isn't only about closing coal plants, though that will be necessary too. It is also about developing aglobal CCS industrythat must by mid-century become bigger than the world's oil extraction industry.

5. Betting on negative emissions

Unfortunately if we were to meet each of those challenges, we would still fall short.

In order to meet limit warming to two degrees the world must go beyond emissions reductions to becomenet carbon neutral by between 2055 and 2070, according to a report from the UN Environment Programme (UNEP) released in November.

Any remaining emissions by 2070 would have to be offset for the world to achieve carbon neutrality. This means the world is betting on negative emissions, where carbon is somehow sucked out of the atmosphere and stored permanently.

To show just how much we are betting on negative emissions, consider this. The IPCC's fifth assessment report found 116 scenarios consistent with keeping warming below two degrees. The vast majority, 101 of the 116 scenarios,rely on negative emissionsafter 2050.

It found another 653 scenarios consistent with warming of 1.7-3.2 degrees by 2100. 235 of these also rely on negative emissions.

There are a number of ways to achieve negative emissions but the most prominent involves attaching CCS technology to power plants burning biomass instead of fossil fuels, a process known asBECCS. If plants, trees or crops are grown at the same time as they are burnt, BECCS could be "carbon negative", meaning it removes carbon dioxide from the air.

Most of the two degrees scenarios identified by the IPCC see BECCS providing over five per cent of global electricity supplies and storing 2-10 gigatonnes of carbon dioxide per year in 2050.

For comparison total world coal emissions stand at about12 gigatonnes per yearwith a capacity of around 2,096 gigawatts. To reach 10 gigatonnes of carbon dioxide removal through BECCS would therefore require around 1,500 gigawatts of BECCS capacity in 2050, equivalent to three-quarters of current coal capacity.

There are currently no operational BECCS plants anywhere in the world.

6. Conclusion

Not everyoneagrees, and more importantly the story doesn't end if we fail to limit warming to two degrees. Vox's Brad Plumer makes acompelling argumentthat at no point does it make sense to declare 'game over' on our climate efforts.

We've already exposed ourselves to increased climate risks by warming the planet by less than a degree. The risks will increase as the planet warms further. Rather than giving up if we fail on two degrees that implies we should do as much as we can to limit warming.

Comments

Gosh, and all of this for only a 66% chance of limiting warming to 2 degrees? It doesn't seem likely, does it.

And AR5, I seem to remember, gave those budgets in ranges, all of which start at zero. That is, it's possible that we've already used up the budget to keep warming to below 2 degrees.

Simon is right, though, 3 degrees is relatively better than 4 degrees and 4 degrees is relatively better than 5 degrees. At what point does it make no difference to our fates? Is 9 degrees better than 10 degrees, for humans (and most other species)?